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is Graeme Obree the only one who's ever used a short BB shell and bearing?
No, there have been others. Only really works for track/fixed, the Qmin is constrained by the need for the chain to reach the top sprocket without hitting the crank on dérailleur bikes. The Walser bikes nicked an extra 10mm by using a special narrow hub, which you can do with a disc wheel. Drive side flange placement on a wire spoked wheel is already close to its limit.
You can theoretically get down to 120mm with a standard 42mm fixed chainline/68mm BB shell, though I'm not aware of any OTP cranks which do. Xav's work identified negligible gains between 120mm and 90mm. To get to 90mm you need elevated chainstays because the gap between the chainring and the tyre is too small to fit anything structurally useful.
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gbj_tester
Following on from the discussion of Q factor upthread, is Graeme Obree the only one who's ever used a short BB shell and bearing? (I think he used a bearing from a washing machine?) There must be a marginal gain to be had for riders with a certain physiology.